1. Field of the Invention
The present invention relates to a solid state drawing process for preparing cavitated oriented polymer compositions that contain fillers.
2. Description of Related Art
Oriented polymer compositions offer benefits in strength over non-oriented polymer compositions. Historically, free draw processes have provided polymeric films with the benefit of increased strength through orientation. Free draw processes are free of physical restraints controlling drawing and offer little control over a final polymer article's shape. Therefore, free draw processes become less desirable as article cross sectional shapes become more complex.
Efforts to establish polymer orientation in articles having cross sections more complex than a thin film eventually led to development of solid state die drawing processes. Solid state die drawing processes draw a polymer composition, at a temperature below the softening temperature of the polymer composition through a drawing die. The drawing die forces the polymer composition to converge towards a specific shape while orienting polymer chains. A drawing die provides more control over final article shape during a drawing process than free drawing processes.
Orientation of filled polymer compositions is becoming of particular interest, especially for forming articles of large cross sections (all cross sectional dimensions greater than 1.5 millimeters). Filler offers numerous benefits, perhaps the most recognized is reduction in raw material cost for the polymer composition. Use of wood fiber fillers in oriented polymer compositions has been particularly popular for fabricating oriented polymer compositions that serve as an alternative to wood decking materials (i.e., composite decking).
Certain fillers also induce cavitation during the solid state drawing process. Cavitation creates void volume in a polymer composition. For example, European Patent 1242220B1 provides an example a polypropylene composition filled with wood filler (composition density of about 1 gram per cubic centimeter) that is drawn at a drawing rate of 48 inches (122 centimeters) per minute to obtain an oriented polymer composition that has a density of 0.59 grams per cubic centimeter. Drawn compositions containing mica filler in polypropylene also reveal void volumes from cavitation of up to 28.5%. (W. R. Newson and F. R. Maine, ORIENTED POLYPROPYLENE COMPOSITIONS MADE WITH MICA, handout from 8th International Conference on Woodfiber-Plastic Composites, Madison, Wis., May 23-25, 2005). Cavitation can be beneficial for reducing oriented polymer composition density. Therefore, fillers may concomitantly reduce the cost and density of an oriented polymer composition. Both of these features are attractive for preparing building materials such as composite decking.
It is desirable to achieve a stable level of cavitation so as to produce boards of consistent density despite process condition fluctuations, such as fluctuations in draw rate, in order to produce oriented filled polymer compositions of having consistent properties (i.e., density).